This invention generally relates to spray dryers and more particularly to methods and equipment for drying particles produced by spray drying.
Spray drying is commonly used in the production of particles for many applications, including food, cosmetics, fertilizers, dyes, and abrasives. Spray drying can be tailored to create a wide spectrum of particle sizes, including microparticles. Spray dried particles are useful in a variety of biomedical and pharmaceutical applications, such as the delivery of therapeutic and diagnostic agents, as described for example in U.S. Pat. No. 5,853,698 to Straub et al., U.S. Pat. No. 5,855,913 to Hanes et al., and U.S. Pat. No. 5,622,657 to Takada et al.
In a typical process for making particles using a spray drying process, a solid forming material, such as a polymer, which is intended to form the bulk of the particle, is dissolved in an appropriate solvent to form a solution. Alternatively, the material can be suspended or emulsified in a non-solvent to form a suspension or emulsion. Other components, such as drugs, diagnostic agents, or pore forming agents, optionally are added at this stage. The solution then is atomized to form a fine mist of droplets. The droplets immediately enter a drying chamber where they contact a drying gas. The solvent is evaporated from the droplets into the drying gas to solidify the droplets, thereby forming particles. The particles then are separated from the drying gas and collected.
In scaling up such a spray drying process, for example from the laboratory or pilot plant scale to the commercial plant scale, certain disadvantages may be encountered. For example, if the drying efficiency is not adequately scaled, the solvent content of the product particles may increase undesirably. While increasing the drying capacity or drying rate should compensate for this insufficient drying, the increased drying rate may induce other problems. For example, it has been observed that increasing the drying rate results in unsuitable particle morphology and/or size distribution for some product particles, such as those having critically defined performance specifications. The change in drying rate may, for instance, alter the way in which the solid-forming material precipitates as the solvent is evaporated, thereby changing the structure (e.g., porosity) of the particle to be out of specification, rendering the particle unable to properly contain and deliver a diagnostic or therapeutic agent. Furthermore, changing the drying rate by reducing the flowrate (and consequently the velocity) of the drying gas may substantially reduce the product yield.
Even in cases where particle morphology and size distribution are less critical, scaling up the drying efficiency may require undesirably large increases in the size of process equipment, such as the drying chamber, drying gas source, and drying gas heater. The drying capacity generally is a function of the drying gas temperature, flowrate, pressure, and solvent composition. Moreover, larger capacity equipment generally requires more plant space. It is desirable to minimize the capital investment and space required to scale up a production process.
Inadequate product drying can also be a problem with known spray drying processes, particularly for some pharmaceutical products which must be dried at low temperatures in order to maintain the stability and/or activity of these materials. Further drying of these materials sensitive to high temperatures can be done using a fluidized bed; however, this process often results in undesirably variable process yields.
Known spray dryers typically are unsuitable for aseptic processing, as they may operate at negative pressure, for example, and may not be designed or constructed to comply with regulatory requirements. In particular, they do not provide a way to completely dry the material aseptically in a sanitizable, closed, and positive-pressure system.
It is therefore an object of the present invention to provide a method and apparatus for effectively drying particles made by spray drying.
It is another object of the present invention to provide a method and apparatus for spray drying that incorporates a drying process providing improved drying of the particles without detrimentally affecting product yield.
It is a further object of the present invention to provide an apparatus for drying spray dried particles that is relatively compact and inexpensive.
It is still another object of the present invention to provide a method and apparatus for spray-drying particles at low temperatures so as to preserve the stability or activity of labile materials.
Improved spray drying methods and equipment are provided. In a preferred embodiment of the method, particles are formed by spraying a solution (or emulsion or solid-in-liquid suspension) of a material into a primary drying chamber and evaporating at least a portion of the solvent (or nonsolvent liquid) sufficient to solidify the particles. The solvent (or nonsolvent) is evaporated into the drying gas in which the particles are entrained. Then, the partially dried particles flow from the primary chamber into a secondary drying apparatus for additional drying. The secondary drying apparatus increases the drying efficiency of the spray dryer system without increasing the drying rate, while minimizing loss in yield The secondary drying apparatus may comprise one or more individual secondary drying apparati which are arranged in series or parallel or a combination thereof. This further improves the product yield, enables the spray drying of particles to take place at multiple temperatures and provides options to maintain biological activity.
The secondary drying apparatus includes tubing having a total length sufficient to increase the contact time between the drying gas and the particles (i.e. increase the residence time) to dry the particles to the extent desired, at a drying capacity or drying rate and temperature which would be too low to provide adequate drying using only the primary drying chamber. For a secondary drying chamber which comprises multiple secondary drying apparati, each secondary drying apparatus corresponds to a different section of tubing. The ratio of the total length of tubing to the length of the primary drying chamber is at least 2:1, and more preferably at least 3:1. Each section of tubing has a cross-sectional area which is substantially smaller than the cross-sectional area of the primary drying chamber, such that the particles move at higher velocity through the sections of tubing to minimize product losses. The ratio of the cross-sectional area of the primary drying chamber to the cross-sectional area of each section of tubing preferably is between about 2:1 and 500:1, more preferably is between about 4:1 and 100:1, and most preferably is about 16:1. In a preferred embodiment, the tubing is stainless steel, and electropolished to 20 RA or smoother, to provide a smooth surface for enhanced particle yield. The tubing preferably is in a compact coil design, for easier transporting and which has minimum space requirements. In another preferred embodiment, the tubing has a jacket to control the temperature of the secondary drying process. The outlet temperature of the primary chamber may be higher, lower or the same as the temperature at which each of the invdividual secondary drying apparati are maintained The primary drying chamber and secondary apparatus or secondary apparati can be integrated into a single unit.
The secondary drying apparati can be independently controlled for temperature and/or have different dimensions (cross-sectional areas and/or lengths), to allow for optimization of drying conditions. For example, in one embodiment, the secondary apparati are set to gradually increase, or decrease, the temperature at which the particles are dried. In another embodiment, the secondary apparati have different dimensions to alter the velocity of the gas which drys particles to accommodate changes in particle aggregation during drying or other properties affecting product yield.
A preferred application for the spray drying process and equipment is in the production of particles between about 1 and 200 xcexcm in diameter, which can be used in the delivery of a diagnostic or therapeutic agent.